For setting the displacement of a hydraulic axial piston unit of the above mentioned type—fixed or variable—an inclined or inclinable swashplate is used. On this non-rotating inclined, swashplate a plurality of working pistons are mounted slide-able in piston slippers which rotate circumferentially on the swashplate. The pistons are movable reciprocally in cylinder bores relative to a cylinder block. Therewith the cylinder block is able to rotate, driving or being driven by a drive shaft defining the rotational axis of the hydraulic axial piston unit. In a preferred case of a variable adjustable hydraulic axial piston unit the swashplate is pivot-able in order to adjust the stroke of the pistons within the cylinder bores. For preventing the slippers from lifting-off from the swashplate a hold down device is provided to hold down the piston slippers in a sliding manner on a sliding surface of the swashplate. Thereby the slipper hold down device is in contact with a radial inner surface with a corresponding matching surface of a guide ball rotatable but fixed, however axially moveable relative to the drive shaft and the cylinder block. Usually the guide ball is mounted pre-stressed in axial direction such that the guide ball presses the hold down device of the piston slippers towards the sliding surface of the swashplate.
At standstill of the hydraulic axial piston unit the piston slippers—in the following only “slippers”—are pressed by the pre-stressing forces of the guide ball on the swashplate thereby having physical contact with the sliding surface of the swashplate. With a rise in the rotational speed of the cylinder block the slippers are lifted-off from the sliding surface, wherein the lifting-off forces which lift-off the slippers from the sliding surface of the swashplate increase with the rotational speed due to the gyroscopic effect. Naturally, the sliding contact between the slippers and the sliding surface of the swashplate must be lubricated in order to reduce friction and wear and to provide a proper function of the hydraulic axial piston unit. In a preferred embodiment the sliding bearing between the slippers and the swashplate is designed as a hydrodynamic bearing which is fed by the working fluid through central bores within the pistons and slippers. In other embodiments this sliding bearing is lubricated by leakage fluid or an oil sump in which the driving unit of the hydraulic axial piston unit rotates.
Irrespective of the kind of the slide bearing, a minimum gap between the sliding surfaces of the slippers and the swashplate must be provided, especially in the case of a hydrodynamic bearing. This means that when the rotation of the driving unit starts the lift-off forces lifting-off the slippers from the sliding surface counteracts against the guide ball forces pressing the slippers onto the sliding surface of the swashplate. These lifting-off forces should already enable low friction conditions at low rotational speeds and should avoid wear and damages to the involved parts and furthermore, should allow a quick response in the rise of the rotational speed of the driving unit. These aspects are favoured when the guide ball forces are low such that friction between the slippers and sliding surface decreases rapidly when the driving unit of the hydraulic axial piston unit starts to rotate.
On the other hand the guide ball pressing forces on the hold down device have to be relatively high if the driving unit of the hydraulic axial piston unit is at high rotational speeds, as the lifting-off forces increase with rising rotational speeds of the driving unit. If at high rotational speeds the pre-stressing forces are too low, the hold down device allows the slippers to lift-off of the sliding surface of the swashplate too much such that the gap between the slippers and the sliding surface is getting too big for a proper sliding bearing. Furthermore, in hydraulic axial piston units with hydrodynamic bearings if the gap between the slippers and the sliding surface is too big the leakage between the slippers and the swashplate increases to an undesired manner.
Furthermore, in the case that the slippers are mounted hydro-dynamically on the sliding surface of the swashplate the lifting-off forces increase with the working pressure too, i.e. with high work load conditions on the driving unit the distance/the gap between the slippers and the sliding surface increases with the increasing working pressure. Therefore the hold down forces of the guide ball must be high enough to maintain the gap small enough for the proper operation of a hydrodynamic bearing and hence to avoid excess of leakage through this gap. In case of hydrodynamic bearing of the slippers, the lifting-off forces caused by the rotational speed augment the lifting-off forces generated by the working pressure. This means, low speed with high working pressure has to be considered when designing the hold down forces generated by the pre-stressed mounting of the guide ball.
In the state of the art there has been a compromise made between these aforementioned opposing requirements, thereby, accepting frequently higher hold down forces at the beginning of rotation in order to keep the gap between the slippers and the swashplate within acceptable limits at high rotational speeds and/or at high work load, i.e. high working pressure. In DE 10 2012 110 853 A1 (JP 2014-095 384 A) a disc spring element is used for this purpose as a hold down device to generate the hold down forces onto the slippers. Thereby, the disc spring element abuts on the guide ball being designed integral with the cylinder block. In DE 1 453 639 C (U.S. Pat. No. 3,191,543 A) a spherical collar is pressed by compression springs against a hold down ring holding down the piston slippers thereby exerting forces of a min. 300 pounds up to 860 pounds are suggested in order to hold the slippers in sliding contact on the sliding surface of the swashplate. In U.S. Pat. No. 4,111,103 a hold down ring is fixed to the swashplate at the radial outer edge of a slipper plate and is used to hold the slipper plate at a fixed distance to the swashplate, thereby allowing the same leakage gap at any operational state of the hydraulic axial piston unit. This gap is previously defined and adjustable by a shim between the slipper plate and the swashplate.